1. Field of the Invention
The present invention relates to a sheet feeding apparatus, and more particularly to a sheet feeding apparatus for feeding a document from a tray onto a platen glass of a copying machine, stopping the document in a specified position on the platen glass and ejecting the document from the platen glass.
2. Description of Related Art
An automatic document feeder attached to an electro-photographic copying machine generally has a pre-feed section which comprises a pick-up roller (or belt) for applying a frictional force to a document to feed the document, a pressing plate for pressing the leading portion of a stack of documents against the feed roller and a leading edge regulation plate for regulating leading edges of documents to align the documents on a tray. Conventionally, the pressing plate is so made that it retreats from its pressing position each time a document has been fed out of the tray. This is for the following reasons. The fed-out document is once stopped and then fed onto a platen glass by a register roller pair. If the pressing plate keeps pressing the stack of documents, when the register roller pair draws and further feeds the document, resistance will be large, whereby the document will be stopped in a wrong position and/or load torque will be large. Also, if the pressing plate keeps pressing the stack of documents, immediately after the trailing edge of a document passes through a separating member, a next document will be fed out of the tray, that is, double-feeding will occur, which may cause size misdetection and sheet jamming. On the other hand, the leading edge regulation plate retreats from a regulating position to a retreating position at the start of feeding of the stack of documents and keeps in the retreating position until the last document is fed out of the tray. If the leading edge regulation plate reciprocates between the regulating position and the retreating position repeatedly during the feeding of the stack, the leading edges of the documents may be damaged, and/or the documents may be fed askew.
Conventionally, the pressing plate and the leading edge regulation plate have separate driving sources (e.g. solenoids). However, providing the separate driving sources is disadvantageous in attaining a space-saving structure. If solenoids, of which speeds of pulling the plungers out (the speed when the loads stop) are high, are used as the driving sources, noise is generated.
An automatic document feeder shall stop a document in a right position on a platen glass for scanning accurately. In a conventional type of automatic document feeder, a conveyer belt is driven by a pulse motor, and the document conveying amount is controlled by detecting the number of pulses. In this type, since clutches are provided in the driving system, the document positioning on the platen glass is not always accurate because of a delay in mechanical response.
An automatic document feeder shall be attachable to different kinds of copying machines. In order to maintain high copy productivity in a combination with any kind of copying machine, the automatic document feeder must have a document conveying speed corresponding to the copying speed of a highest-speed type copying machine. However, if the automatic document feeder has the same document conveying speed even when it is attached to a low-speed type copying machine, noise will be remarkable, and the lives of the components will be shortened.
Incidentally, as modes of stopping a document in a specified position (scanning position) on the platen glass, the following two modes have been adopted: a scale mode wherein a document stops when its leading edge comes into contact with a scale disposed at a scanning reference point; and a pulse control mode wherein the document conveying amount attained by the conveyer belt is detected (generally, the number of pulses driving the motor is detected) and driving of the conveyer belt is controlled accordingly.
The scale mode has an advantage that the document positioning is very accurate because the scale directly stops the document. However, if the document is thin, in the scale mode, trouble such as bending of the leading edge of the document and/or sheet jamming may occur. On the other hand, the pulse control mode has an advantage that even if the document is thin, the trouble will not occur. However, in the pulse control mode, the document positioning is not so accurate as in the scale mode because of slipping of the document and/or a response delay of the driving mechanism.
It is preferred that an automatic document feeder has a document conveying speed corresponding to the copying speed of the copying machine. More specifically, if a change of documents on the platen glass is completed within a time while the optical system of the copying machine is returning to its home position after scanning, the copy productivity can be maintained in 100%. Practically, however, actions of the automatic document feeder are not only for the document change but also for other purposes, and there are some actions which lower the copying efficiency of the copying machine.
In connection with a copying system composed of a copying machine and an automatic document feeder, recently, a two-in-one mode has been developed. In the two-in-one mode, two documents are set on the platen glass side by side and copied onto a single copy sheet. If this mode is further developed so as to make a set of copies of the same sheet size from a set of documents containing different sizes, for example, to copy two A4-sized documents onto a single A3-sized copy sheet and to copy a single A3-sized document onto an A3-sized copy sheet, it will be very advantageous. However, conventionally, if documents of different sizes are in a stack to be copied in the two-in-one mode, sheet jamming is judged in connection with a large size document, and the operation is discontinued. The copying machine also cannot respond to documents of different sizes fed in the two-in-one mode.
A count mode wherein the automatic document feeder feeds a stack of documents once before copying so as to count the documents has been developed. Since the operation in the count mode does not directly contribute to copying, preferably, the operation is finished as speedily as possible. In a type of automatic document feeder wherein each document is stopped on the platen glass with its leading edge positioned at a scanning reference point which is located in a downstream portion of the platen glass, the interval between documents is unnecessarily large for the count mode, and the conveyer belt idles unnecessarily. Accordingly, the counting in this type takes a time. Also in a type of automatic document feeder wherein a scanning reference point is located in an upstream of the platen glass, if the distance between the scanning reference point and the register roller pair is long, the interval between documents is unnecessarily large for the count mode, and the same problem will occur.
Additionally, a pre-step mode has been developed. In the pre-step mode, if documents to be fed are of a size smaller than a half of the platen glass, a first document and a second document are set in a scanning position where the leading edge of a document is positioned at a scanning reference point which is located in the downstream end of the platen glass and in a pre-step position where the leading edge of a document is positioned at an intermediate reference point which is located substantially in the center of the platen glass respectively, and further a third document is fed to an entrance of the platen glass. In the pre-step mode, compared with an ordinary feeding mode, only a half time is required for a change of documents, and this contributes to maintenance of a high copy productivity in a high-speed copying system. However, conventionally, if documents of different sizes are in a stack fed in the pre-step mode, sheet jamming is judged in connection with a large size document, and the operation is discontinued. The copying machine also cannot respond to the large size document fed in the pre-step mode.
In the two-in-one mode described above, two documents set on the platen glass are ejected from the platen glass onto an ejected-document tray with no space between the documents. In this state, the latter document may hit the former document, which causes page disorder on the ejected-document tray.